The optical signals carried in a wireless optical communication system travel, at least in part, through free space. Important in the design of such systems are the interface components used to launch light signals into free space from a fixed medium, such as a fiber optic cable, and to receive light signals into such a cable from free space. These interface components can include optical receivers, transmitters, transceivers (devices that both transmit and receive optical signals), and couplers that couple light between transmission media, such as the interface between air and an optical fiber.
Systems designers strive to make the transfer characteristics of the interface components as efficient and accurate as possible. This is due, in part, to the challenges associated with coupling light signals directly into a fiber, as compared to coupling light signals to or from an optical detector or emitter. The objective lenses of these components, or the lenses or lens system that first receives the light rays from free space or from the fiber, typically include highly-polished lenses made of glass. Because of the design constraints involved, lens manufacturers tend to use similar designs, including using similar materials, such as glass, shapes, such as circular, and rigidity. As such, the objective lens products that are commercially available for use in wireless optical communication systems tend be similar in size, cost, and performance allowing little choice in the design of a system.
FIG. 1 illustrates a lensed optical receiver for use in a wireless optical communication system, in which an optical signal 1 first enters a circular objective lens 2. The objective lens 2 can focus the optical signal 1 onto a second lens 3 configured to align the incoming rays of light that form the optical signal 1. The aligned optical signal can be passed through a wavelength specific filter 4 that filters out non-signal-carrying light rays from the aligned beam. The wavelength response of the filter 4 can have an angular dependence, in which case the collimating lens 3 should be included in the system to optimize performance of the receiver. The aligned and filtered optical signal can be passed through a focusing lens 5, that focuses the signal of an optical detector 6.
The optical detector 6 can be a photodiode, phototransistor, avalanche photodiode, photomultiplier tube, piezoelectric device, charged coupled device (CCD), or any other device that can produce a signal in response to an optical stimulus. The optical detector 6 can generate a signal that is proportional to the light energy of the optical signal 1. The generated signal can be a voltage, a current, or another light signal. A line driver 7 can be used to drive the generated signal down a wire or optical cable 8. The optics and circuitry of the receiver can be housed or enclosed within a water tight, and preferably hermetically sealed, enclosure 9. The enclosure 9 can help to maintain an alignment of the optics and to protect the circuitry from exposure to the environment.
The design of an optical transmitter for use in a wireless optical communication system can be similar to the optical receiver shown in FIG. 1 because of the bipolar nature of light as it travels through an optical system. FIG. 2 illustrates a lensed optical transmitter, in which an information-carrying signal can be provided through a wire or optical fiber cable 15. Circuitry 14 can be provided to receive, amplify, condition, and process the voltage, current, or optical signal carried over the cable 15. The circuitry 14 can be configured to drive an optical emitter 13 that can convert the information-carrying signal into optical energy. The emitter can be a laser, light emitting diode or other similar light-emitting device.
A secondary lens 12 can be included in the transmitter to magnify the light emitted by the emitter 13. Unlike the receiver shown in FIG. 1, an optical filter is typically not needed in an a transmitter, as the optical emitter can be designed to emit only the desired wavelengths of light. An objective lens 11 can align the rays of light 10 of the emitted beam, such that the beam can be transmitted over a range to a distant receiver (not shown). As with the receiver shown in FIG. 1, the optics and circuitry of the transmitter can be protected by an enclosure 16 that is watertight, and preferably hermetically sealed. Because of the typically large size of the objective lenses 3, 11 and the precision that should be maintained in the alignment of the optics of the receiver and transmitter, such enclosures can be bulky and expensive to manufacture.